Distinct Functional Roles of Narrow and Broadband High-Gamma Activities in Human Primary Somatosensory Cortex
In previous studies, higher (broadband) and lower (narrowband) components of high-gamma (HG) activity (approximately from 50 to 150 Hz) have different functions and origins in the primary visual cortex (V1). However, in the primary somatosensory cortex (S1), it is unknown whether those are similarly segregated. Furthermore, the origin and functional role of S1 HG activity still remain unclear. Here, we investigate their roles by measuring neural activity during vibrotactile and texture stimuli in humans. Also, to estimate their origins, S1 layer-specific HG activity was measured in rats during somatosensory stimulation. In the human experiment, with texture stimulation, the lower HG activity (LHG, 50-70 Hz) in S1 represents the intensity of the sustained mechanical stimulus. In the vibrotactile experiment, the higher HG (HHG, 70 -150 Hz) activity in S1 depended on the ratio of low and high mechanical frequencies with its pattern being a mixture of neural activity for low and high mechanical frequencies. Furthermore, 8 texture types could be classified using power values of HHG activity, while the classification using LHG activity showed poor performance. In the rat experiment, we found that both HHG and LHG activities are highest in the somatosensory input layer (layer IV), similar to previous visual cortex studies. Interestingly, analysis of spike-triggered LFP (stLFP) revealed significant HG oscillations during pressure stimulation with the stLFP HG power most significant in layer IV, suggesting that both LHG and HHG activities are closely related to the neuronal firing in layer IV. In summary, LHG activity represents the intensity of tactile sensation, while HHG activity represents the detail of the surface geometry of objects interacting with skin. Additionally, low and high mechanical frequencies are processed in parallel in S1. Finally, both HHG and LHG originated in layer IV of S1.